The present invention generally relates to micromachining processes and devices formed thereby. More particularly, this invention relates to a process of forming a micromachined tube suitable for a microfluidic device.
Processes for fabricating resonant mass flow and density sensors using silicon micromachining techniques are disclosed in commonly-assigned U.S. Pat. Nos. 6,477,901 and 6,647,778. As used herein, micromachining is a technique for forming very small elements by bulk etching a substrate (e.g., a silicon wafer), and/or by surface thin-film etching, the latter of which generally involves depositing a thin film (e.g., polysilicon or metal) on a sacrificial layer (e.g., oxide layer) on a substrate surface and then selectively removing portions of the sacrificial layer to free the deposited thin film. In the processes disclosed in U.S. Pat. Nos. 6,477,901 and 6,647,778, wafer bonding and etching techniques are used to produce a micromachined tube supported above a surface of a substrate. The tube can be vibrated at resonance, by which the flow rate, density, and/or other properties or parameters of a fluid flowing through the tube can be measured.
According to one embodiment of U.S. Pat. No. 6,477,901, a tube is formed using p-type doped layers and selective etching techniques. The doped layers form the walls of the tube, and therefore determine and limit the size of the tube walls as well as the cross-sectional dimensions of the tube. According to another embodiment of U.S. Pat. No. 6,477,901, a tube is formed with the use of a silicon-on-insulator (SOI) wafer. The buried oxide layer of the SOI wafer is used as an etch stop in a manner that determines and can limit the thickness of the tube. Another consideration of this embodiment is the higher cost of SOI silicon wafers as compared to standard silicon wafers. In co-pending U.S. patent application Ser. No. 11/161,901, a process is disclosed by which an epitaxial wafer can be employed to avoid the cost of using a SOI wafer. Though of considerably lower cost than SOI silicon wafers, epitaxial silicon wafers are significantly more expensive than standard silicon wafers.
In view of the above, while well suited for producing micromachined tubes for microfluidic devices, it would be advantageous if other micromachining processes were available that avoid the size restraints of previous processes, as well as potentially simplify processing and reduce cost and processing time.